Abstract: A blue composite film is assembled on a glass substrate using a self-standing casting method based on the electrostatic attraction and intercalation of methylene blue (MB) with DNA. The resulting cast film is characterized by means of UV-visible absorption spectroscopy, fluorescence spectroscopy and scanning electron microscopy, and its casting mechanism mediated by DNA is further explored. The results suggest that MB can intercalate into DNA and simultaneously induce the condensation of DNA to form a uniform MB-DNA composite film. The aggregated forms of MB can be disassociated into monomer or dimer forms based on the dispersion of DNA, revealed by their photoabsorption properties. This present study provides a powerful basis for constructing DNA-mediated optoelectronic functional devices.